Electrical earth borehole logging is well known and various devices and techniques have been described. In an electrical investigation of a borehole, current from an electrode is introduced in the formation from a tool inside the borehole. If this current is maintained constant, the voltage measured at a monitor electrode is proportional to the resistivity of the earth formation being investigated. If the current is varied to maintain constant the voltage measured at a monitor electrode, the current is inversely proportional to the resistivity of the earth formation being investigated. If both voltage and current are allowed to vary, their ratio is proportional to the resistivity of the earth formation being investigated. Substantial advances have been made in such electrical investigations by using electrodes whose currents are focused by other electrodes and thus determine the resistivity of the formation at a desired distance from the borehole wall surface. One technique and device for focused electrical investigation may be as described and shown in the U.S. Pat. Nos. 2,712,629 to Doll and 2,750,557 to Bricaud. In U.S. Pat. No. 2,712,629 pad mounted sets of electrodes are described as each formed of a central survey electrode surrounded at spaced intervals by continuous guard electrodes embedded in segmented recesses. In U.S. Pat. No. 2,750,557 the pad mounted electrodes are formed of electrically directly connected segments or buttons.
In the U.S. Pat. No. 3,521,154 to Maricelli a plurality of survey electrodes are mounted on a single pad as a composite focusing electrode, with a pair of the survey electrodes aligned along the direction of travel of the tool along the borehole and one survey electrode horizontally displaced to provide a technique for effectively improving the signal to noise ratio of the resistivity measurements.
Techniques for investigating the earth formation with larger arrays of measuring electrodes have been proposed. See, for example, the U.S. Pat. No. 2,930,969 to Baker and Canadian Pat. No. 685,727 to Mann et al. The Baker patent proposed a plurality of electrodes, each of which was formed of buttons which are electrically joined by flexible wires with buttons and wires embedded in the surface of a collapsible tube. The Canadian patent proposes an array of small electrode buttons either mounted on a tool or a pad and each of which introduces in sequence a separately measurable survey current for an electrical investigation of the earth formation. The electrode buttons are placed as shown in the Canadian patent in a horizontal plane with circumferential spacings between electrodes and a device for sequentially exciting and measuring a survey current from the electrodes is described.
A technique has been proposed in an article "Reservoir Evaluation of Fractured Cretaceous Carbonates in South Texas" by J. Beck et al, presented at the SPWLA Eighteenth Annual Logging Symposium of June 5-8, 1977 to identify fractures in the wall of a borehole using a resistivity dip meter tool. The tool employs several pads each of which has a survey electrode from which current is introduced into the borehole wall. The presence of a fracture may be identified by noting a deviation between the survey currents from different pads. Such survey current deviation may indicate a fracture by virtue, for example, of the invasion of higher conducting mud into the fracture. In one specific technique a small survey electrode is used on each of four pads to provide an indication of vertical fractures when the tool is rotated as it is moved along the borehole.
Although these known techniques provide useful information of the earth formation surrounding a borehole, their surface area coverage does not enable the detection of resistivity anomalies of sufficiently fine detail and accuracy useful in determining the presence of fractures and thin bed layers or their orientations.